It is known that grain oriented electrical steel sheets having crystal grains in accord with {110}<001> orientation (hereinafter, “Goss orientation”) through secondary recrystallization annealing exhibit superior magnetic properties (e.g. see JP 540-15644B). As indices of magnetic properties of the grain oriented electrical steel sheets, magnetic flux density B8 at a magnetic field strength of 800 A/m and iron loss (per kg) W17/50 of the steel sheet when it is magnetized to 1.7 T in an alternating magnetic field with an excitation frequency of 50 Hz, are mainly used.
Further, it has been a common practice in manufacturing grain oriented electrical steel sheets to use precipitates called inhibitors to induce differences of grain boundary mobility during final annealing so that the crystal grains preferentially grow only in the Goss orientation.
For example, JP 540-15644B discloses a method of using AlN and MnS, while JP 551-13469B discloses a method of using MnS and MnSe. Both have been put into practical use industrially.
Since those methods using inhibitors require a uniform and fine precipitate distribution of inhibitors as an ideal state, it is necessary to heat a slab before hot rolling to 1300° C. or higher. As such high temperature slab heating is performed, excessive coarsening occurs in the crystal structure of the slab. With such coarsening, the orientation of the slab structure tends to grow in {100}<011> orientation which is a stable orientation of hot rolling, which greatly impedes grain growth during secondary recrystallization, thereby leading to serious deterioration of magnetic properties.
For the purpose of reducing the above coarse slab structure, JP H03-10020A discloses a technique of obtaining uniformly recrystallized microstructures by performing high reduction rolling at a temperature range of 1280° C. or higher in the first pass of rough rolling, thereby facilitating generation of recrystallization nuclei from grain boundaries of a grains.
For the purpose of recrystallization of the surface layer of the hot rolled sheet, JP H02-101121A discloses a technique of performing hot rolling with a rolling reduction of 40% to 60% in a temperature range of 1050° C. to 1150° C. using the rolls having surface roughness of 4 μmRa to 8 μmRa, to increase the amount of shear strain in the surface layer of the hot rolled sheet.
Further, JP S61-34117A discloses a technique to grow only highly oriented secondary recrystallized grains, by subjecting a silicon steel slab containing 0.01 wt % to 0.06 wt % of C to high reduction rolling of 40% or more in the first pass of finish hot rolling, and afterward to light reduction rolling of 30% or less per 1 pass so that Goss orientation grains existing in the surface layer of the hot rolled sheet increase. The Goss orientation grains lead to the increased amount of Goss orientation grains in the surface layer after primary recrystallization annealing through a so called “structure memory mechanism”.
JP H03-10020A discloses high reduction rolling at a temperature of 1280° C. or higher in rough hot rolling. However, as a technical concept, this is originally high reduction rolling in an a single phase region, and there existed a problem that an (α+γ) dual phase is formed even at a temperature of 1280° C. or higher depending on compositions, so that sufficiently uniform recrystallized microstructures cannot be obtained.
Further, according to JP H02-101121A, shear strain in the surface layer of the hot rolled sheet increases by controlling finish hot rolling condition. However, recrystallization is hard to occur in the center layer in sheet thickness direction of a steel sheet where shear strain is difficult to be introduced, and there still remained a problem in facilitating recrystallization in the center layer.
Further, it is assumed that JP H02-101121A and JP S61-34117A mainly focus on high reduction rolling in a temperature range of high γ phase volume fraction. However, since the temperature range of the maximum γ phase volume fraction greatly varies depending on the material compositions, there was a problem that, when using certain compositions, high reduction rolling is performed in a temperature range out of the temperature range of maximum γ phase volume fraction, which results in an insufficient improving effect of magnetic properties.